How Does Plant Architecture and Water Use Efficiency Impact Crop Yields?

• The effects of different plant architectures on population-level evapotranspiration are not well-known.
• Plant architecture influences population structure, thereby altering the microclimate.
• Soil water use can be reduced through shading to minimize evaporation and by reducing canopy temperature.

Around 97-99% of the water absorbed by plants is lost as transpiration. As water shortages are increasing and restricting crop yields worldwide, the water-use efficiency of cultivars must be improved to maintain and increase yields with reduced water usage. Optimizing plant canopy traits can reduce water use at the population level. This article examines how scientists aim to utilize plant architecture to regulate population transpiration, thereby enhancing yield.

Reducing Water Use

According to UNESCO, nearly 70% of fresh water withdrawals are used for agriculture. To address the issue of water shortages, it is essential to reduce the amount of water used by crops. Developing cultivars with higher water-use efficiency is one way to make better use of available water resources.

Crop water-use efficiency (WUE) is the amount of biomass produced per unit of water absorbed by crops. WUE is influenced by water use than by biomass production. Hence, further increases in WUE can come from water-saving plant traits.

Most of the water lost through transpiration occurs through leaf stomata. Transpiration is thus connected to photosynthesis, as carbon dioxide enters the plants through their stomata. Reduction in stomatal conductance will lower transpiration and water loss, but also limit photosynthesis and biomass accumulation.

Thus, future cultivars must maintain higher stomatal conductance and photosynthesis even in water-stress periods.

Population Transpiration

Loss of water in farms is due to evapotranspiration, where evaporation from the soil accounts for around 20-30%, and transpiration from the population-scale accounts for 65-70%. The proportion depends on the crop stage, as initially, at sowing, 100% of water loss is due to evaporation; however, when the full crop cover is developed, 90% of water loss is due to crop transpiration.

Plant traits and environmental factors influence crop population transpiration.

• Environmental factors include irrigation levels, soil moisture availability, climate, soil type, and management practices.
• Crop traits: Features such as growth stage, genetics, canopy, and leaf traits can be crucial.

Stomatal conductance will also depend on the time of day, season, and the microclimate. Transpiration is highest when air temperatures are high and lower in the morning and evening when temperatures are lower. Transpiration can be only one-third in winter compared to other seasons.

Similarly, microclimates that are cooler and more humid also influence transpiration: the same stomatal conductance or opening results in less transpiration in a cooler microclimate.

Various plant parameters can themselves contribute to creating a cooler microclimate, which in turn affects evapotranspiration. One of them is the canopy structure of the crop population, which involves the exchange of heat between the plants and the atmosphere. Population structure, in turn, is affected by plant architecture.

Plant architecture

Plant architecture is the three-dimensional organization of shoot and root components. The aboveground plant architecture that determines population structure includes plant height, the arrangement of branches, the size, shape, orientation, and position of leaves and flowers. Plant architecture varies by species and is influenced by both genetic factors and environmental conditions.

Scientists have utilized plant architecture modification for decades, and this trait has contributed significantly to the successes of the Green Revolution in enhancing crop productivity. However, the influence of different plant architectures on population transpiration remains largely unknown.

Plant architecture determines light interception that can influence photosynthesis, air permeability, and canopy temperature. Hence, plant architecture and population structure alter the microclimate, which enhances crop water use efficiency (WUE).

• Canopy temperature depression, and
• Reduction in soil evaporation and water losses

Table 1: “Plant architecture attributes of the genotypes Jinmai 47 and Jing 411,” Huang et al. (2023). (Credit: https://www.mdpi.com/2073-4395/13/3/742)

Canopy Temperature Depression

The cooling effect of transpiration determines the canopy temperature and can reduce it to a level lower than that of the surrounding air. This effect is called the canopy temperature depression (CTD).

CTD is closely correlated with the canopy structure, which affects the movement of solar radiation through the canopy, the heat exchange between the canopy and the air, and the water use by plants.

The plant architecture is plastic and can vary within a species, and thus, CTD can also vary within a species. The range of CTD among various cultivars is due to differences in transpiration rates. CTD was closely related to the amounts of transpiration in sugarbeets, rice, wheat, and potatoes. Within a single species like wheat, there is a range of possible CTD values from positive to negative. CTD is positive when air temperatures are higher and canopy temperature is lower due to transpirational cooling. CTD is negative when air temperatures are lower and there is a water deficit. CTD of wheat has been shown to differ in the same climate, but with different management practices that affect population structure, primarily by altering density.

Canopy structure needs to be regulated to break the correlation between population transpiration and canopy temperature depression. The population structure’s effect on light and air movement determines the extent of CTD achieved. When there is more air movement, the heat penetrates the canopy, lowering CTD, but encouraging more water loss or transpiration. Canopy structures that do not allow good airflow could restrict the distribution of heat and keep canopies cooler.

A recent study published by Huang et al. (2023) examined the transpiration effects of two contrasting wheat plant architectures and population structure, whose parameters are summarized in Table 1. One population structure type had a compact canopy, upright growth, smaller, darker leaves, and a shorter stem. This plant architecture facilitated heat exchange, resulting in low CTD and high transpiration.

A loose canopy, flat leaf orientation, larger bright green leaves, and higher plant height characterized the other population structure. The flat leaves prevented air movement and heat exchange, resulting in a cooler canopy and a high CTD, but lower water use. Planting in high-density clumps instead of uniform planting further strengthened the population structure and cooling effects during the vegetative stage, as well as yield, due to increased photosynthesis. Paradoxically, stomatal conductance and population transpiration were similar in the two plant architecture types. Still, water use at the population level with higher planting density was lower for crops with loose canopies and flat leaves.

The color of leaves can also help. Light colored leaves in wheat and barley produced a stronger positive CTD than dark green leaves. Moreover, lighter leaves allow more light percolation to lower leaves, increasing photosynthetic rate and boosting crop yield.

Reducing Evaporation

Canopy coverage over the soil can also regulate the evaporation of soil water. Smaller canopies with upright leaves expose more soil and lead to more evaporation losses of soil water. A canopy that offers more soil coverage through shading will prevent evaporation, as seen in the loose flat-leaved wheat canopies. Reducing evapotranspiration provides more water for crop transpiration, which is linearly correlated with yield.

Selecting cultivars with loose canopies, flat leaves, and high water-use efficiency can be a means of conserving water without compromising yield.

Promising Avenues of Research

Increasing yield by controlling evapotranspiration through the regulation of plant architecture represents a novel approach to optimizing water use efficiency (WUE). Research on this theme has just begun, and more crop species and cultivars must be tested to fill the gaps in our understanding of the correlation between crop population structure and transpiration. Similar research will need to delve into leaf, canopy, and gas exchanges, which require tools designed for on-site measurements. CID Bio-Science Inc. offers portable precision tools that cover several aspects of the research:

• CI-202 and CI-203 Laser Leaf Area Meters to measure leaf size and shape.
• CI-340 Handheld Photosynthesis System is an infrared gas analyzer for stomatal conductance, transpiration and photosynthetic rate measurements.
• CI-110 Plant Canopy Imager to quantify canopy cover.

Contact us at CID Bio-Science Inc. to find out how we can help in furthering your population transpiration research.

Sources

Huang, G., Zhang, X., Wang, Z., Li, Y., Liu, X., Guo, R., Gu, F., Liu, E., Li, S., Zhong, X., & Mei, X. (2023). Plant Architecture Influences the Population Transpiration and Canopy Temperature in Winter Wheat Genotypes. Agronomy, 13(3), 742. https://doi.org/10.3390/agronomy13030742

Irmak, S. (2009). Estimating Crop Evapotranspiration from Reference Evapotranspiration and Crop Coefficients. Retrieved from https://extensionpubs.unl.edu/publication/g1994/2017/html/view

Reinhardt, D., & Kuhlemeier, C. (2002). Plant architecture. EMBO reports, 3(9), 846–851. https://doi.org/10.1093/embo-reports/kvf177

Runkle, E. (2023, Sept). The Importance of Transpiration. Retrieved from https://www.canr.msu.edu/floriculture/uploads/files/Transpiration.pdf

Vegetti A. C. (2023). Editorial: Plant architecture: structure, development, evolution, and molecular/genetic regulation. Frontiers in plant science, 14, 1211224. https://doi.org/10.3389/fpls.2023.1211224